Bolts and nut are used to create the clamp force necessary to fasten two components together. The technician must understand the grade of the bolt and the type of nut that is used with that bolt, as using the wrong bolts or nuts can lead to a mechanical failure of that component. A mechanical failure occurs if either the bolt break or the nut strips, both of which can result from not using the proper nuts or bolts, or applying the wrong amount of torque to those bolts.
Bolts are categorized by there grades, which represent the strength of the bolt. Grade 2 bolts are the weakest and are composed of low carbon steel; they are typicaly used in light duty applications like shed frameworks. Grade 5 bolts are medium in strength and often feature three radial lines on the bolt head.
Types of Bolts and Nuts and How to Tighten Them
Grade 8 bolts is made of alloy steel and are high in strength; they are used in heavy duty applications like suspension systems or machinery. Metric bolts use a different strength grading system then bolts that are labeled with the grade numbers. Metric class 8.8 bolts are used in standard automotive repairs.
Metric class 10.9 and 12.9 bolts is used in heavy duty vehicles with high tensile requirements. The grade of the bolt should match the grade of the nut that is used with it; otherwise, the nut may strip prior to the bolt fracturing. Nuts come in a variety of different designs and uses.
One of the most common designs for nuts are plain hex nuts. Nylock nuts contains a nylon insert that prevents the nut from loosening due to vibration. Jam nuts are thin nuts that are used to hold another nut in place; they do not add to the height of the bolt assembly.
Castle nuts have slots in the nut that allow for the insertion of a cotter pin to secure the bolt. Finally, flange nuts have a built in washer; they are used for applications where the nut should not press against the mating component directly. Nuts should be selected based off the specific application for which they will be used, rather than what is available in the maintenance shops storage bin.
The torque that is applied to bolts will determine the clamp force that is created by the nut and bolt. The torque that is required for a bolt of a specific size can differ based upon the grade of the bolt. For instance, a 1/2 inch bolt of grade 5 can require 75 ft-lbs of torque to achieve the necessary clamp force, but a 1/2 inch bolt of grade 8 may require 106 ft-lbs of torque.
Lubricating the bolt or nut can reduce the amount of torque that is applied to a bolt; dry threads can have a friction factor of 0.20, but lubrication can reduce that factor to 0.15. Similarly, anti-seize lubricants can reduce the friction factor of threaded component to 0.12. Using lubrication will require a reduction of the amount of torque that is applied to the bolt; using too much torque can lead to the bolt snapping.
If a bolt snaps, that is a mechanical failure of the component. In order to tighten the bolts properly, the technician should use the appropriate tools. Combination wrenches are used for fastening bolts that are exposed to odd angles in the component.
Sockets are used for repetitive applications of turning force. Click type torque wrenches allow the technician to ensure that the proper amount of torque is applied to the bolt. Impact wrenches are strong tools that are used in situations where a great deal of torque is required, but they are too strong for fine tasks; they can easily apply too much torque to the bolts.
Finally, a tightening sequence should be used to even out the clamp force that the bolt exerts upon the components. A tightening sequence may use a star pattern or spiral tightening pattern. Additionally, the technician should tighten the bolts in stages; 30%, 70%, and 100% of the required torque should be applied to each bolt in that order.
Prior to tightening the bolts, a few steps should be performed to ensure the integrity of the component. First, the technician should clean the threads on the component. Additionally, the thread pitch of the bolt should match that of the nut, and the threads must engage with one another sufficiently; the bolt should be threaded into the nut for a distance that is at least the diameter of the bolt.
The grade markings on the bolts should be examined to ensure they are of the proper grade for the component to which they are to be installed. Finally, the torque wrench should be calibrated. If the bolts are over-torqued, they could snap.
If the bolts are incorrectly tightened, the threads can be damaged. These errors could of been avoided by focusing upon the correct amount of clamp force that must be created by the bolts.
